Method to drive spatially separate resonant structure with spatially distinct plasma secondaries using a single generator and switching elements

ABSTRACT

A plasma reactor for processing a workpiece, the plasma reactor comprising an enclosure, a workpiece support within the enclosure facing an overlying portion of the enclosure, the workpiece support and the overlying portion of the enclosure defining a process region therebetween extending generally across the diameter of said wafer support, the enclosure having a first and second pairs of openings therethrough, the two openings of each of the first and second pairs being near generally opposite sides of said workpiece support, a first hollow conduit outside of the process region and connected to the first pair of openings, providing a first torroidal path extending through the conduit and across the process region, a second hollow conduit outside of the process region and connected to the second pair of openings, providing a second torroidal path extending through the conduit and across the process region, first and second plasma source power applicators inductively coupled to the interiors of the first and second hollow conduits, respectively, each of the first and second plasma source power applicators being capable of maintaining a plasma in a respective one of the first and second torroidal paths, an RF power generator providing an RF output current, a current switching network connected between the RF power generator and the first and second plasma source power applicators for applying respective periodic time segments of RF output current to respective ones of said first and second plasma source power applicators.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 09/636,435 nowU.S. Pat. No. 6,494,986, filed Aug. 11, 2000 entitled, “EXTERNALLYEXCITED MULTIPLE TORROIDAL PLASMA SOURCE,” By Hiroji Hanawa, et al.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention concerns plasma reactors used in processing workpieces inthe manufacturing of items such as microelectronic circuits, flat paneldisplays and the like, and in particular plasma sources therefor.

2. Background Art

The trend in microelectronic circuits toward ever increasing densitiesand smaller feature sizes continues to make plasma processing of suchdevices more difficult. For example, the diameter of contact holes hasbeen reduced while the hole depth has increased. During plasma-enhancedetching of a dielectric film on a silicon wafer, for example, the etchselectivity of the dielectric material (e.g. silicon dioxide) tophotoresist must be sufficient to allow the etch process to etch acontact hole whose diameter is ten to fifteen times its depth, withoutappreciably disturbing the photoresist mask defining the hole. This taskis made even more difficult because the recent trend toward shorterwavelength light for finer photolithography requires a thinnerphotoresist layer, so that the dielectric-to-photoresist etchselectivity must be greater than ever. This requirement is more readilymet using processes having relatively low etch rates, such as dielectricetch processes employing a capacitively coupled plasma. The plasmadensity of a capacitively coupled plasma is relatively less than that ofan inductively coupled plasma, and the capacitively coupled plasma etchprocess exhibits good dielectric-to-photoresist etch selectivity. Theproblem with the capacitively coupled process is that it is slow andtherefore relatively less productive. Another problem that arises insuch etch processes is non-uniform plasma distribution.

In order to increase productivity or etch rate, higher density plasmashave been employed. Typically, the high density plasma is an inductivelycoupled plasma. However, the process precursor gases tend to dissociatemore rapidly in such a high density plasma, creating a higher plasmacontent of free fluorine, a species which reduces the etch selectivityto photoresist. To reduce this tendency, fluoro-carbon process gasessuch as CF₂ are employed which dissociate in a plasma intofluorine-containing etchant species and one or more polymer specieswhich tend to accumulate on non-oxide containing surfaces such asphotoresist. This tends to increase etch selectivity. The oxygen in theoxygen-containing dielectric material promotes the pyrolization of thepolymer over the dielectric, so that the polymer is removed, allowingthe dielectric material to be etched while the non-oxygen containingmaterial (e.g., the photoresist) continues to be covered by the polymerand therefore protected from the etchant. The problem is that theincrease in contact opening depth and decrease in photoresist thicknessto accommodate more advanced device designs has rendered the highdensity plasma process more likely to harm the photoresist layer duringdielectric etching. As the plasma density is increased to improve etchrate, a more polymer-rich plasma must be used to protect the non-oxygencontaining material such as photoresist, so that the rate of polymerremoval from the oxygen-containing dielectric surfaces slowsappreciably, particularly in small confined regions such as the bottomof a narrow contact opening. The result is that, while the photoresistmay be adequately protected, the possibility is increased for the etchprocess to be blocked by polymer accumulation once a contact openingreaches a certain depth. Typically, the etch stop depth is less than therequired depth of the contact opening so that the device fails. Thecontact opening may provide connection between an upper polysiliconconductor layer and a lower polysilicon conductor layer through anintermediate insulating silicon dioxide layer. Device failure occurs,for example, where the etch stop depth is less than the distance betweenthe upper and lower polysilicon layers. Alternatively, the possibilityarises of the process window for achieving a high density plasma withoutetch stop becoming too narrow for practical or reliable application tothe more advanced device designs such as those having contact openingswith aspect ratios of 10:1 or 15:1.

What would be desirable at present is a reactor that has the etch rateof an inductively coupled plasma reactor (having a high density plasma)with the selectivity of a capacitively coupled reactor. It has beendifficult to realize the advantages of both types of reactors in asingle machine led reactor.

One problem with high density inductively coupled plasma reactors,particularly of the type having an overhead coil antenna facing thewafer or workpiece, is that as the power applied to the coil antenna isincreased to enhance the etch rate, the wafer-to-ceiling gap must besufficiently large so that the power is absorbed in the plasma regionwell above the wafer. This avoids a risk of device damage on the waferdue to strong RF fields. Moreover, for high levels of RF power appliedto the overhead coil antenna, the wafer-to-ceiling gap must berelatively large, and therefore the benefits of a small gap cannot berealized.

If the ceiling is a semiconductive window for the RF field of aninductively coupled reactor, or a conductive electrode of a capacitivelycoupled reactor, then one benefit of a small wafer-to-ceiling gap is anenhanced electric potential or ground reference that the ceiling couldprovide across the plane of the wafer at a relatively small gap distance(e.g., on the order of 1 or 2 inches).

Therefore, it would be desirable to have a reactor not only having theselectivity of a capacitively coupled reactor with the ion density andetch rate of an inductively coupled reactor, but further having none ofthe conventional limitations on the wafer-to-ceiling gap length otherthan a fundamental limit, such as the plasma sheath thickness, forexample. It would further be desirable to have a reactor having theselectivity of a capacitively coupled reactor and the etch rate of aninductively coupled reactor in which the ion density and etch rate canbe enhanced without necessarily increasing the applied RF plasma sourcepower.

SUMMARY OF THE DISCLOSURE

A plasma reactor for processing a workpiece includes a workpiece supportwithin an enclosure facing an overlying portion of the enclosure, theworkpiece support and the overlying portion of the enclosure defining aprocess region therebetween extending generally across the diameter ofthe wafer support. An array of pair openings extend through the vacuumenclosure, the openings in each pair being near generally opposite sidesof the workpiece support. An array of hollow conduits outside of thevacuum chamber are connected to respective ones of the pairs ofopenings, whereby to provide respective closed torroidal paths forplasma, each of the respective closed torroidal paths extending outsideof the vacuum chamber through a respective one of the array of conduitsand extending inside the vacuum chamber between a respective pair of theopenings across the wafer surface. Respective source power applicatorsare coupled to respective ones of the conduits. A plasma source power RFgenerator provides a cyclical current output. A current switchingnetwork is connected between the source power RF generator and each ofthe respective source power applicators, for dividing each cycle of thecyclical current output into respective time segments and applying therespective time segments to respective ones of the source powerapplicators.

The current switching network in one implementation is diode-controlledcurrent divider. In another implementation the current switching networkincludes respective transistor switches separately connected between theRF power generator and respective ones of the plasma source powerapplicators and a source of mutually exclusive enabling signalscontrolling the respective transistor switches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment that maintains an overheadtorroidal plasma current path.

FIG. 2 is a side view of an embodiment corresponding to the embodimentof FIG. 1.

FIG. 3 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in wafer-to-ceiling gapdistance.

FIG. 4 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF bias power applied tothe workpiece.

FIG. 5 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF source power appliedto the coil antenna.

FIG. 6 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in reactor chamber pressure.

FIG. 7 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in partial pressure of aninert diluent gas such as Argon.

FIG. 8 is a graph illustrating the degree of dissociation of process gasas a function of source power for an inductively coupled reactor and fora reactor of the present invention.

FIG. 9 illustrates a variation of the embodiment of FIG. 1.

FIGS. 10 and 11 illustrate a variation of the embodiment of FIG. 1 inwhich a closed magnetic core is employed.

FIG. 12 illustrates another embodiment of the invention in which atorroidal plasma current path passes beneath the reactor chamber.

FIG. 13 illustrates a variation of the embodiment of FIG. 10 in whichplasma source power is applied to a coil wound around a distal portionthe closed magnetic core.

FIG. 14 illustrates an embodiment that establishes two paralleltorroidal plasma currents.

FIG. 15 illustrates an embodiment that establishes a plurality ofindividually controlled parallel torroidal plasma currents.

FIG. 16 illustrates a variation of the embodiment of FIG. 15 in whichthe parallel torroidal plasma currents enter and exit the plasma chamberthrough the vertical side wall rather than the ceiling.

FIG. 17A illustrates an embodiment that maintains a pair of mutuallyorthogonal torroidal plasma currents across the surface of theworkpiece.

FIG. 17B illustrates the use of plural radial vanes in the embodiment ofFIG. 17A.

FIGS. 18 and 19 illustrate an embodiment of the invention in which thetorroidal plasma current is a broad belt that extends across a wide pathsuitable for processing large wafers.

FIG. 20 illustrates a variation of the embodiment of FIG. 18 in which anexternal section of the torroidal plasma current path is constricted.

FIG. 21 illustrates a variation of the embodiment of FIG. 18 employingcylindrical magnetic cores whose axial positions may be adjusted toadjust ion density distribution across the wafer surface.

FIG. 22 illustrates a variation of FIG. 21 in which a pair of windingsare wound around a pair of groups of cylindrical magnetic cores.

FIG. 23 illustrates a variation of FIG. 22 in which a single commonwinding is wound around both groups of cores.

FIGS. 24 and 25 illustrate an embodiment that maintains a pair ofmutually orthogonal torroidal plasma currents which are wide beltssuitable for processing large wafers.

FIG. 26 illustrates a variation of the embodiment of FIG. 25 in whichmagnetic cores are employed to enhance inductive coupling.

FIG. 27 illustrates a modification of the embodiment of FIG. 24 in whichthe orthogonal plasma belts enter and exit the reactor chamber throughthe vertical side wall rather than through the horizontal ceiling.

FIG. 28A illustrates an implementation of the embodiment of FIG. 24which produces a rotating torroidal plasma current.

FIG. 28B illustrates a version of the embodiment of FIG. 28A thatincludes magnetic cores.

FIG. 29 illustrates a preferred embodiment of the invention in which acontinuous circular plenum is provided to enclose the torroidal plasmacurrent.

FIG. 30 is a top sectional view corresponding to FIG. 29.

FIGS. 31A and 31B are front and side sectional views corresponding toFIG. 30.

FIG. 32 illustrates a variation of the embodiment 29 employing threeindependently driven RF coils underneath the continuous plenum facing at120 degree intervals.

FIG. 33 illustrates a variation of the embodiment of FIG. 32 in whichthe three RF coils are driven at 120 degree phase to provide anazimuthally rotating plasma.

FIG. 34 illustrates a variation of the embodiment of FIG. 33 in which RFdrive coils are wound around vertical external ends of respectivemagnetic cores whose opposite ends extend horizontally under the plenumat symmetrically distributed angles.

FIG. 35 is an version of the embodiment of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the embodiment of FIG. 20.

FIG. 36 is a version of the embodiment of FIG. 24 but employing a pairof magnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

FIG. 37 is an embodiment corresponding to that of FIG. 35 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

FIG. 38 is an embodiment corresponding to that of FIG. 38 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

FIG. 39 is an embodiment corresponding to that of FIG. 35 in which theexternal conduits join together in a common plenum 3910.

FIG. 40 is an embodiment corresponding to that of FIG. 36 in which theexternal conduits join together in a common plenum 4010.

FIG. 41 is an embodiment corresponding to that of FIG. 37 in which theexternal conduits join together in a common plenum 4110.

FIG. 42 is an embodiment corresponding to that of FIG. 38 in which theexternal conduits join together in a common plenum 4210.

FIG. 43 is an embodiment corresponding to that of FIG. 17 in which theexternal conduits join together in a common plenum 4310.

FIG. 44A is a perspective view of an embodiment corresponding to theembodiment of FIG. 24 with two external reentrant conduits forming twotorroidal plasma paths, in which only a single RF power generatorfurnishes power to the source power applicators for the two externalconduits through a diode-controlled current divider circuit.

FIG. 44B is a cross-sectional side view corresponding to FIG. 44A.

FIGS. 45A and 45B are contemporaneous timing diagrams of RF currentsapplied to source power applicators of respective external reentrantconduits of the embodiment of FIG. 44A.

FIG. 46 illustrates an embodiment corresponding to FIG. 44A in which thediode controlled current divider circuit of FIG. 44A is replaced by aclocked MOSFET switching circuit.

FIGS. 47A, 47B and 47C are contemporaneous timing diagrams of,respectively, the RF generator output current and a pair ofcomplementary clock signals controlling the MOSFET switching circuit.

FIG. 48 illustrates an embodiment corresponding to FIG. 44A employing amodification of the diode-controlled current divider circuit of FIG. 44Ain which a transformer is eliminated.

FIG. 49 illustrates an embodiment corresponding to FIG. 48 employingplural parallel diodes in the diode-controlled current divider circuitfor enhanced current-carrying capacity.

FIG. 50A illustrates a reactor for processing two wafers simultaneouslyin the same chamber on respective wafer support pedestals using acombination of two reactors of the type of FIG. 24 in a single chamberfor a total of four external reentrant conduits arranged in pairs ofmutually transverse conduits over respective ones of the two wafersupport pedestals.

FIG. 50B is a cross-sectional side corresponding to FIG. 50A.

FIG. 51 illustrates an embodiment corresponding to FIG. 50A in whichonly a single RF power generator applies power to the source powerapplicators of the four external reentrant conduits through a switchingnetwork current divider.

FIG. 52 is a timing diagram illustrating how the switching networkcurrent divider of FIG. 51 divides the output of the single RF powergenerator of FIG. 51 into four periodic phases.

FIGS. 53A, 53B, 53C and 53D illustrate the waveforms of the RF currentsapplied to the source power applicators of the four external reentrantconduits of FIG. 51.

FIG. 54 illustrates an exemplary implementation of the switching networkof FIG. 51.

FIGS. 55 and 56A, 56B, 56C, 56D are contemporaneous timing diagramsillustrating, respectively, the source power RF generator output currentand four mutually exclusive clock signals controlling the switchingnetwork of FIG. 54.

FIG. 57 is an embodiment corresponding to FIG. 50A in which only two RFpower generators of different frequencies or phases apply power to thesource power applicators of the four external reentrant conduits througha diode-controlled current divider.

FIGS. 58A and 58B illustrate the RF current waveforms of the two sourcepower RF generators in the embodiment of FIG. 55.

FIGS. 59A and 59B illustrate the RF current waveforms applied torespective source power applicators of one of the pair of mutuallytransverse conduits of FIG. 55.

FIGS. 60A and 60B illustrate the RF current waveforms applied torespective source power applicators of the other pair of mutuallytransverse conduits of FIG. 55.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Overview of the Plasma Reactor Chamber:

Referring to FIG. 1, a plasma reactor chamber 100 enclosed by acylindrical side wall 105 and a ceiling 110 houses a wafer pedestal 115for supporting a semiconductor wafer or workpiece 120. A process gassupply 125 furnishes process gas into the chamber 100 through gas inletnozzles 130 a-130 d extending through the side wall 105. A vacuum pump135 controls the pressure within the chamber 100, typically holding thepressure below 0.5 milliTorr (mT). A halftorroidal hollow tube enclosureor conduit 150 extends above the ceiling 110 in a half circle. Theconduit 150, although extending externally outwardly from ceiling 110,is nevertheless part of the reactor and forms a wall of the chamber.Internally it shares the same evacuated atmosphere as exists elsewherein the reactor. In fact, the vacuum pump 135, instead of being coupledto the bottom of the main part of the chamber as illustrated in FIG. 1,may instead be coupled to the conduit 150, although this is notpresently preferred. The conduit 150 has one open end 150 a sealedaround a first opening 155 in the reactor ceiling 110 and its other end150 b sealed around a second opening 160 in the reactor ceiling 110. Thetwo openings or ports 155, 160 are located on generally opposite sidesof the wafer support pedestal 115. The hollow conduit 150 is reentrantin that it provides a flow path which exits the main portion of thechamber at one opening and re-enters at the other opening. In thisspecification, the conduit 150 may be described as being half-torroidal,in that the conduit is hollow and provides a portion of a closed path inwhich plasma may flow, the entire path being completed by flowing acrossthe entire process region overlying the wafer support pedestal 115.Notwithstanding the use of the term Atorroidal@, the trajectory of thepath as well as the cross-sectional shape of the path or conduit 150 maybe circular or non-circular, and may be square, rectangular or any othershape either a regular shape or irregular.

The external conduit 150 may be formed of a relatively thin conductorsuch as sheet metal, but sufficiently strong to withstand the vacuumwithin the chamber. In order to suppress eddy currents in the sheetmetal of the hollow conduit 150 (and thereby facilitate coupling of anRF inductive field into the interior of the conduit 150), an insulatinggap 152 extends across and through the hollow conduit 150 so as toseparate it into two tubular sections. The gap 152 is filled by a ring154 of insulating material such as a ceramic in lieu of the sheet metalskin, so that the gap is vacuum tight. A second insulating gap 153 maybe provided, so that one section of the conduit 150 is electricallyfloating. A bias RF generator 162 applies RF bias power to the waferpedestal 115 and wafer 120 through an impedance match element 164.

Alternatively, the hollow conduit 150 may be formed of a non-conductivematerial instead of the conductive sheet metal. The non-conductivematerial may be a ceramic, for example. In such an alternativeembodiment, neither gap 152 or 153 is required.

An antenna 170 such as a winding or coil 165 disposed on one side of thehollow conduit 150 and wound around an axis parallel to the axis ofsymmetry of the half-torroidal tube is connected through an impedancematch element 175 to an RF power source 180. The antenna 170 may furtherinclude a second winding 185 disposed on the opposite side of the hollowconduit 150 and wound in the same direction as the first winding 165 sothat the magnetic fields from both windings add constructively.

Process gases from the chamber 100 fill the hollow conduit 150. Inaddition, a separate process gas supply 190 may supply process gasesdirectly in to the hollow conduit 150 through a gas inlet 195. The RFfield in the external hollow conduit 150 ionizes the gases in the tubeto produce a plasma. The RF field induced by the circular coil antenna170 is such that the plasma formed in the tube 150 reaches through theregion between the wafer 120 and the ceiling 110 to complete a torroidalpath that includes the half-torroidal hollow conduit 150. As employedherein, the term “torroidal” refers to the closed and solid nature ofthe path, but does not refer or limit its cross-sectional shape ortrajectory, either of which may be circular or non-circular or square orotherwise. Plasma circulates through the complete torroidal path orregion which may be thought of as a closed plasma circuit. The torroidalregion extends across the diameter of the wafer 120 and, in certainembodiments, has a sufficient width in the plane of the wafer so that itoverlies the entire wafer surface.

The RF inductive field from the coil antenna 170 includes a magneticfield which itself is closed (as are all magnetic fields), and thereforeinduces a plasma current along the closed torroidal path described here.It is believed that power from the RF inductive field is absorbed atgenerally every location along the closed path, so that plasma ions aregenerated all along the path. The RF power absorption and rate of plasmaion generation may vary among different locations along the closed pathdepending upon a number of factors. However, the current is generallyuniform along the closed path length, although the current density mayvary. This current alternates at the frequency of the RF signal appliedto the antenna 170. However, since the current induced by the RFmagnetic field is closed, the current must be conserved around thecircuit of the closed path, so that the amount of current flowing in anyportion of the closed path is generally the same as in any other portionof the path. As will be described below, this fact is exploited in theinvention to great advantage.

The closed torroidal path through which the plasma current flows isbounded by plasma sheaths formed at the various conductive surfacesbounding the path. These conductive surfaces include the sheet metal ofthe hollow conduit 150, the wafer (and/or the wafer support pedestal)and the ceiling overlying the wafer. The plasma sheaths formed on theseconductive surfaces are charge-depleted regions produced as the resultof the charge imbalance due to the greater mobility of the low-massnegative electrons and the lesser mobility of the heavy-mass positiveions. Such a plasma sheath has an electric field perpendicular to thelocal surface underlying the sheath. Thus, the RF plasma current thatpasses through the process region overlying the wafer is constricted byand passes between the two electric fields perpendicular to the surfaceof the ceiling facing the wafer and the surface of the wafer facing thegas distribution plate. The thickness of the sheath (with RF biasapplied to the workpiece or other electrode) is greater where theelectric field is concentrated over a small area, such as the wafer, andis less in other locations such as the sheath covering the ceiling andthe large adjoining chamber wall surfaces. Thus, the plasma sheathoverlying the wafer is much thicker. The electric fields of the waferand ceiling/gas distribution plate sheaths are generally parallel toeach other and perpendicular to the direction of the RF plasma currentflow in the process region.

When RF power is first applied to the coil antenna 170, a dischargeoccurs across the gap 152 to ignite a capacitively coupled plasma fromgases within the hollow conduit 150. Thereafter, as the plasma currentthrough the hollow conduit 150 increases, the inductive coupling of theRF field becomes more dominant so that the plasma becomes an inductivelycoupled plasma. Alternatively, plasma may be initiated by other means,such as by RF bias applied to the workpiece support or other electrode.

In order to avoid edge effects at the wafer periphery, the ports 155,160 are separated by a distance that exceeds the diameter of the wafer.For example, for a 12 inch diameter wafer, the ports 155, 160 are about16 to 22 inches apart. For an 8 inch diameter wafer, the ports 155, 160are about 10 to 16 inches apart.

Advantages of the Invention:

A significant advantage is that power from the RF inductive field isabsorbed throughout the relatively long closed torroidal path (i.e.,long relative to the gap length between the wafer and the reactorceiling), so that RF power absorption is distributed over a large area.As a result, the RF power in the vicinity of the wafer-to-ceiling gap(i.e., the process region 121 best shown in FIG. 2, not to be confusedwith the insulating gap 152) is relatively low, thus reducing thelikelihood of device damage from RF fields. In contrast, in priorinductively coupled reactors, all of the RF power is absorbed within thenarrow wafer-to-ceiling gap, so that it is greatly concentrated in thatregion. Moreover, this fact often limits the ability to narrow thewafer-to-ceiling gap (in the quest of other advantages) or,alternatively, requires greater concentration of RF power in the regionof the wafer. Thus, the invention overcomes a limitation of longstanding in the art. This aspect enhances process performance byreducing residency time of the reactive gases through a dramaticreduction in volume of the process region or process zone overlying thewafer, as discussed previously herein.

A related and even more important advantage is that the plasma densityat the wafer surface can be dramatically increased without increasingthe RF power applied to the coil antenna 170 (leading to greaterefficiency). This is accomplished by reducing the cross-sectional areaof the torroidal path in the vicinity of the pedestal surface and wafer120 relative to the remainder of the torroidal path. By so constrictingthe torroidal path of the plasma current near the wafer only, thedensity of the plasma near the wafer surface is increasedproportionately. This is because the torroidal path plasma currentthrough the hollow conduit 150 must be at least nearly the same as theplasma current through the pedestal-to-ceiling (wafer-to-ceiling) gap.

A significant difference over the prior art is that not only is the RFfield remote from the workpiece, and not only can ion density beincreased at the wafer surface without increasing the applied RF field,but the plasma ion density and/or the applied RF field may be increasedwithout increasing the minimum wafer-to-ceiling gap length. Formerly,such an increase in plasma density necessitated an increase in thewafer-to-ceiling gap to avoid strong fields at the wafer surface. Incontrast, in the present invention the enhanced plasma density isrealized without requiring any increase in the wafer-to-ceiling gap toavoid a concomitant increase in RF magnetic fields at the wafer surface.This is because the RF field is applied remotely from the wafer andmoreover need not be increased to realize an increase in plasma densityat the wafer surface. As a result, the wafer-to-ceiling gap can bereduced down to a fundamental limit to achieve numerous advantages. Forexample, if the ceiling surface above the wafer is conductive, thenreducing the wafer-to-ceiling gap improves the electrical or groundreference provided by the conductive ceiling surface. A fundamentallimit on the minimum wafer-to-ceiling gap length is the sum of thethicknesses of the plasma sheaths on the wafer surface and on theceiling surface.

A further advantage of the invention is that because the RF inductivefield is applied along the entire torroidal path of the RF plasmacurrent (so that its absorption is distributed as discussed above), thechamber ceiling 110, unlike with most other inductively poweredreactors, need not function as a window to an inductive field andtherefore may be formed of any desired material, such as a highlyconductive and thick metal, and therefore may comprise a conductive gasdistribution plate as will be described below, for example. As a result,the ceiling 110 readily provides a reliable electric potential or groundreference across the entire plane of the pedestal or wafer 120.

Increasing the Plasma Ion Density:

One way of realizing higher plasma density near the wafer surface byreducing plasma path cross-sectional area over the wafer is to reducethe wafer-to-ceiling gap length. This may be accomplished by simplyreducing the ceiling height or by introducing a conductive gasdistribution plate or showerhead over the wafer, as illustrated in FIG.2. The gas distribution showerhead 210 of FIG. 2 consists of a gasdistribution plenum 220 connected to the gas supply 125 andcommunicating with the process region over the wafer 120 through pluralgas nozzle openings 230. The advantage of the conductive showerhead 210is two-fold: First, by virtue of its close location to the wafer, itconstricts the plasma path over the wafer surface and thereby increasesthe density of the plasma current in that vicinity. Second, it providesa uniform electrical potential reference or ground plane close to andacross the entire wafer surface.

Preferably, in order to avoid arcing across the openings 230, eachopening 230 is relatively small, on the order of a millimeter (preferredhole diameter is approximately 0.5 mm). The spacing between adjacentopenings may be on the order of a several millimeters.

The conductive showerhead 210 constricts the plasma current path ratherthan providing a short circuit through itself because a plasma sheath isformed around the portion of the showerhead surface immersed in theplasma. The sheath has a greater impedance to the plasma current thanthe space between the wafer 120 and the showerhead 210, and thereforeall the plasma current goes around the conductive showerhead 210.

It is not necessary to employ a showerhead (e.g., the showerhead 210) inorder to constrict the torroidal plasma current or path in the vicinityof the process region overlying the wafer. The path constriction andconsequent increase in plasma ion density in the process region may beachieved without the showerhead 210 by similarly reducing thewafer-to-ceiling height. If the showerhead 210 is eliminated in thismanner, then the process gases may be supplied into the chamber interiorby means of conventional gas inlet nozzles (not shown).

One advantage of the showerhead 210 is that different mixtures ofreactive and inert process gas ratios may be introduced throughdifferent orifices 230 at different radii, in order to finely adjust theuniformity of plasma effects on photoresist, for example. Thus, forexample, a greater proportion of inert gas to reactive gas may besupplied to the orifices 230 lying outside a median radius while agreater proportion of reactive gas to inert gas may be supplied to theorifices 230 within that median radius.

As will be described below, another way in which the torroidal plasmacurrent path may be constricted in the process region overlying thewafer (in order to increase plasma ion density over the wafer) is toincrease the plasma sheath thickness on the wafer by increasing the RFbias power applied to the wafer support pedestal. Since as describedpreviously the plasma current across the process region is confinedbetween the plasma sheath at the wafer surface and the plasma sheath atthe ceiling (or showerhead) surface, increasing the plasma sheaththickness at the wafer surface necessarily decreases the cross-sectionof the portion of the torroidal plasma current within process region,thereby increasing the plasma ion density in the process region. Thus,as will be described more fully later in this specification, as RF biaspower on the wafer support pedestal is increased, plasma ion densitynear the wafer surface is increased accordingly.

High Etch Selectivity at High Etch Rates:

The invention solves the problem of poor etch selectivity whichsometimes occurs with a high density plasma. The reactor of FIGS. 1 and2 has a silicon dioxide-to-photoresist etch selectivity as high as thatof a capacitively coupled plasma reactor (about 7:1) while providinghigh etch rates approaching that of a high density inductively coupledplasma reactor. It is believed that the reason for this success is thatthe reactor structure of FIGS. 1 and 2 reduces the degree ofdissociation of the reactive process gas, typically a fluorocarbon gas,so as to reduce the incidence of free fluorine in the plasma region overthe wafer 120. Thus, the proportion of free fluorine in the plasmarelative to other species dissociated from the fluorocarbon gas isdesirably reduced. Such other species include the protective carbon-richpolymer precursor species formed in the plasma from the fluorocarbonprocess gas and deposited on the photoresist as a protective polymercoating. They further include less reactive etchant species such as CFand CF₂ formed in the plasma from the fluorocarbon process gas. Freefluorine tends to attack photoresist and the protective polymer coatingformed thereover as vigorously as it attacks silicon dioxide, thusreducing oxide-to-photoresist etch selectivity. On the other hand, theless reactive etch species such as CF₂ or CF tend to attack photoresistand the protective polymer coating formed thereover more slowly andtherefore provide superior etch selectivity.

It is believed the reduction in the dissociation of the plasma speciesto free fluorine is accomplished in the invention by reducing theresidency time of the reactive gas in the plasma. This is because themore complex species initially dissociated in the plasma from thefluorocarbon process gas, such as CF₂ and CF are themselves ultimatelydissociated into simpler species including free fluorine, the extent ofthis final step of dissociation depending upon the residency time of thegas in the plasma. The term “residency time” or “residence time” asemployed in this specification corresponds generally to the average timethat a process gas molecule and the species dissociated from the thatmolecule are present in the process region overlying the workpiece orwafer. This time or duration extends from the initial injection of themolecule into the process region until the molecule and/or itsdissociated progeny are pass out of the process region along the closedtorroidal path described above that extends through the processing zone.

As stated above, the invention enhances etch selectivity by reducing theresidency time in the process region of the fluorocarbon process gas.The reduction in residency time is achieved by constricting the plasmavolume between the wafer 120 and the ceiling 110.

The reduction in the wafer-to-ceiling gap or volume has certainbeneficial effects. First, it increases plasma density over the wafer,enhancing etch rate. Second, residency time falls as the volume isdecreased. As referred to above, the small volume is made possible inthe present invention because, unlike conventional inductively coupledreactors, the RF source power is not deposited within the confines ofthe process region overlying the wafer but rather power deposition isdistributed along the entire closed torroidal path of the plasmacurrent. Therefore, the wafer-to-ceiling gap can be less than a skindepth of the RF inductive field, and in fact can be so small as tosignificantly reduce the residency time of the reactive gases introducedinto the process region, a significant advantage.

There are two ways of reducing the plasma path cross-section andtherefore the volume over the wafer 120. One is to reduce thewafer-to-showerhead gap distance. The other is to increase the plasmasheath thickness over the wafer by increasing the bias RF power appliedto the wafer pedestal 115 by the RF bias power generator 162, as brieflymentioned above. Either method results in a reduction in free fluorinecontent of the plasma in the vicinity of the wafer 120 (and consequentincrease in dielectric-to-photoresist etch selectivity) as observedusing optical emission spectroscopy (OES) techniques.

There are three additional methods of the invention for reducing freefluorine content to improve etch selectivity. One method is to introducea non-chemically reactive diluent gas such as argon into the plasma.Preferably, the argon gas is introduced outside and above the processregion by injecting it directly into the hollow conduit 150 from thesecond process gas supply 190, while the chemically reactive processgases (fluorocarbon gases) enter the chamber only through the showerhead210. With this advantageous arrangement, the argon ions, neutrals, andexcited neutrals propagate within the torroidal path plasma current andthrough the process region across the wafer surface to dilute the newlyintroduced reactive (e.g., fluorocarbon) gases and thereby effectivelyreduce their residency time over the wafer. Another method of reducingplasma free fluorine content is to reduce the chamber pressure. Afurther method is to reduce the RF source power applied to the coilantenna 170.

FIG. 3 is a graph illustrating a trend observed in the invention inwhich the free fluorine content of the plasma decreases as thewafer-to-showerhead gap distance is decreased. FIG. 4 is a graphillustrating that the free fluorine content of the plasma is decreasedby decreasing the plasma bias power applied to the wafer pedestal 115.FIG. 5 is a graph illustrating that plasma free fluorine content isreduced by reducing the RF source power applied to the coil antenna 170.FIG. 6 is a graph illustrating that the free fluorine content is reducedby reducing chamber pressure. FIG. 7 is a graph illustrating that plasmafree fluorine content is reduced by increasing the diluent (Argon gas)flow rate into the tubular enclosure 150. The graphs of FIGS. 3-7 aremerely illustrative of plasma behavioral trends inferred from numerousOES observations and do not depict actual data.

Wide Process Window of the Invention:

Preferably, the chamber pressure is less than 0.5 T and can be as low as1 mT. The process gas may be C₄F₈ injected into the chamber 100 throughthe gas distribution showerhead at a flow rate of about 15 cc/m with 150cc/m of Argon, with the chamber pressure being maintained at about 20mT. Alternatively, the Argon gas flow rate may be increased to 650 cc/mand the chamber pressure to 60 mT. The antenna 170 may be excited withabout 500 Watts of RF power at 13 MHz. The wafer-to-showerhead gap maybe about 0.3 inches to 2 inches. The bias RF power applied to the waferpedestal may be 13 MHz at 2000 Watts. Other selections of frequency maybe made. The source power applied to the coil antenna 170 may be as lowas 50 KHz or as high as several times 13 MHz or higher. The same is trueof the bias power applied to the wafer pedestal.

The process window for the reactor of FIGS. 1 and 2 is far wider thanthe process window for a conventional inductively coupled reactor. Thisis illustrated in the graph of FIG. 8, showing the specific neutral fluxof free fluorine as a function of RF source power for a conventionalinductive reactor and for the reactor of FIGS. 1 and 2. For theconventional inductively coupled reactor, FIG. 8 shows that the freefluorine specific flux begins to rapidly increase as the source powerexceeds between 50 and 100 Watts. In contrast, the reactor of FIGS. 1and 2 can accept source power levels approaching 1000 Watts before thefree fluorine specific flux begins to increase rapidly. Therefore, thesource power process window in the invention is nearly an order ofmagnitude wider than that of a conventional inductively coupled reactor,a significant advantage.

Dual Advantages of the Invention:

The constriction of the torroidal plasma current path in the vicinity ofthe wafer or workpiece produces two independent advantages without anysignificant tradeoffs of other performance criteria: (1) the plasmadensity over the wafer is increased without requiring any increase inplasma source power, and (2) the etch selectivity to photoresist orother materials is increased, as explained above. It is believed that inprior plasma reactors it has been impractical if not impossible toincrease the plasma ion density by the same step that increases etchselectivity. Thus, the dual advantages realized with the torroidalplasma source of the present invention appear to be a revolutionarydeparture from the prior art.

Other Preferred Embodiments:

FIG. 9 illustrates a modification of the embodiment of FIG. 1 in whichthe side antenna 170 is replaced by a smaller antenna 910 that fitsinside the empty space between the ceiling 110 and the hollow conduit150. Preferably, the antenna 910 is a single coil winding centered withrespect to the hollow conduit 150.

FIGS. 10 and 11 illustrate how the embodiment of FIG. 1 may be enhancedby the addition of a closed magnetically permeable core 1015 thatextends through the space between the ceiling 110 and the hollow conduit150. The core 1015 improves the inductive coupling from the antenna 170to the plasma inside the hollow conduit 150.

Impedance match may be achieved without the impedance match circuit 175by using, instead, a secondary winding 1120 around the core 1015connected across a tuning capacitor 1130. The capacitance of the tuningcapacitor 1130 is selected to resonate the secondary winding 1120 at thefrequency of the RF power source 180. For a fixed tuning capacitor 1130,dynamic impedance matching may be provided by frequency tuning and/or byforward power servoing.

FIG. 12 illustrates an embodiment of the invention in which a hollowtube enclosure 1250 extends around the bottom of the reactor andcommunicates with the interior of the chamber through a pair of openings1260, 1265 in the bottom floor of the chamber. A coil antenna 1270follows along side the torroidal path provided by the hollow tubeenclosure 1250 in the manner of the embodiment of FIG. 1. While FIG. 12shows the vacuum pump 135 coupled to the bottom of the main chamber, itmay just as well be coupled instead to the underlying conduit 1250.

FIG. 13 illustrates a variation of the embodiment of FIGS. 10 and 11, inwhich the antenna 170 is replaced by an inductive winding 1320surrounding an upper section of the core 1015. Conveniently, the winding1320 surrounds a section of the core 1015 that is above the conduit 150(rather than below it). However, the winding 1320 can surround anysection of the core 1015.

FIG. 14 illustrates an extension of the concept of FIG. 13 in which asecond hollow tube enclosure 1450 runs parallel to the first hollowconduit 150 and provides a parallel torroidal path for a secondtorroidal plasma current. The tube enclosure 1450 communicates with thechamber interior at each of its ends through respective openings in theceiling 110. A magnetic core 1470 extends under both tube enclosures150, 1450 and through the coil antenna 170.

FIG. 15 illustrates an extension of the concept of FIG. 14 in which anarray of parallel hollow tube enclosures 150 a, 150 b, 150 c, 150 dprovide plural torroidal plasma current paths through the reactorchamber. In the embodiment of FIG. 15, the plasma ion density iscontrolled independently in each individual hollow conduit 150 a-d by anindividual coil antenna 170 a-d, respectively, driven by an independentRF power source 180 a-d, respectively. Individual cylindrical open cores1520 a-1520 d may be separately inserted within the respective coilantennas 170 a-d. In this embodiment, the relative center-to-edge iondensity distribution may be adjusted by separately adjusting the powerlevels of the individual RF power sources 180 a-d.

FIG. 16 illustrates a modification of the embodiment of FIG. 15 in whichthe array of tube enclosures 150 a-d extend through the side wall 105 ofthe reactor rather than through the ceiling 110. Another modificationillustrated in FIG. 16 is the use of a single common magnetic core 1470adjacent all of the tube enclosures 150 a-d and having the antenna 170wrapped around it so that a single RF source excites the plasma in allof the tube enclosures 150 a-d.

FIG. 17A illustrates a pair of orthogonal tube enclosures 150-1 and150-2 extending through respective ports in the ceiling 110 and excitedby respective coil antennas 170-1 and 170-2. Individual cores 1015-1 and1015-2 are within the respective coil antennas 170-1 and 170-2. Thisembodiment creates two mutually orthogonal torroidal plasma currentpaths over the wafer 120 for enhanced uniformity. The two orthogonaltorroidal or closed paths are separate and independently powered asillustrated, but intersect in the process region overlying the wafer,and otherwise do not interact. In order to assure separate control ofthe plasma source power applied to each one of the orthogonal paths, thefrequency of the respective RF generators 180 a, 180 b of FIG. 17 aredifferent, so that the operation of the impedance match circuits 175 a,175 b is decoupled. For example, the RF generator 180 a may produce anRF signal at 11 MHz while the RF generator 180 b may produce an RFsignal at 12 MHz. Alternatively, independent operation may be achievedby offsetting the phases of the two RF generators 180 a, 180 b.

FIG. 17B illustrates how radial vanes 181 may be employed to guide thetorroidal plasma currents of each of the two conduits 150-1, 150-2through the processing region overlying the wafer support. The radialvanes 181 extend between the openings of each conduit near the sides ofthe chamber up to the edge of the wafer support. The radial vanes 181prevent diversion of plasma from one torroidal path to the othertorroidal path, so that the two plasma currents only intersect withinthe processing region overlying the wafer support.

Embodiments Suitable for Large Diameter Wafers:

In addition to the recent industry trends toward smaller device sizesand higher device densities, another trend is toward greater waferdiameters. For example, 12-inch diameter wafers are currently enteringproduction, and perhaps larger diameter wafers will be in the future.The advantage is greater throughput because of the large number ofintegrated circuit die per wafer. The disadvantage is that in plasmaprocessing it is more difficult to maintain a uniform plasma across alarge diameter wafer. The following embodiments of the present inventionare particularly adapted for providing a uniform plasma ion densitydistribution across the entire surface of a large diameter wafer, suchas a 12-inch diameter wafer.

FIGS. 18 and 19 illustrate a hollow tube enclosure 1810 which is a wideflattened rectangular version 1850 of the hollow conduit 150 of FIG. 1that includes an insulating gap 1852. This version produces a wide“belt” of plasma that is better suited for uniformly covering a largediameter wafer such as a 12-inch diameter wafer or workpiece. The widthW of the tube enclosure and of the pair of openings 1860, 1862 in theceiling 110 preferably exceeds the wafer by about 5% or more. Forexample, if the wafer diameter is 10 inches, then the width W of therectangular tube enclosure 1850 and of the openings 1860, 1862 is about11 inches. FIG. 20 illustrates a modified version 1850′ of therectangular tube enclosure 1850 of FIGS. 18 and 19 in which a portion1864 of the exterior tube enclosure 1850 is constricted. However, theunconstricted version of FIGS. 18 and 19 is preferred.

FIG. 20 further illustrates the optional use of focusing magnets 1870 atthe transitions between the constricted and unconstricted portions ofthe enclosure 1850. The focusing magnets 1870 promote a better movementof the plasma between the constricted and unconstricted portions of theenclosure 1850, and specifically promote a more uniform spreading out ofthe plasma as it moves across the transition between the constrictedportion 1864 and the unconstricted portion of the tube enclosure 1850.

FIG. 21 illustrates how plural cylindrical magnetic cores 2110 may beinserted through the exterior region 2120 circumscribed by the tubeenclosure 1850. The cylindrical cores 2110 are generally parallel to theaxis of symmetry of the tube enclosure 1850. FIG. 22 illustrates amodification of the embodiment of FIG. 21 in which the cores 2110 extendcompletely through the exterior region 2120 surrounded by the tubeenclosure 1850 are replaced by pairs of shortened cores 2210, 2220 inrespective halves of the exterior region 2120. The side coils 165, 186are replaced by a pair of coil windings 2230, 2240 surrounding therespective core pairs 2210, 2220. In this embodiment, the displacement Dbetween the core pairs 2210, 2220 may be changed to adjust the iondensity near the wafer center relative to the ion density at the wafercircumference. A wider displacement D reduces the inductive couplingnear the wafer center and therefore reduces the plasma ion density atthe wafer center. Thus, an additional control element is provided forprecisely adjusting ion density spatial distribution across the wafersurface. FIG. 23 illustrates a variation of the embodiment of FIG. 22 inwhich the separate windings 2230, 2240 are replaced by a single centerwinding 2310 centered with respect to the core pairs 2210, 2220.

FIGS. 24 and 25 illustrate an embodiment providing even greateruniformity of plasma ion density distribution across the wafer surface.In the embodiment of FIGS. 24 and 25, two torroidal plasma current pathsare established that are transverse to one another and preferably aremutually orthogonal. This is accomplished by providing a second widerectangular hollow enclosure 2420 extending transversely and preferablyorthogonally relative to the first tube enclosure 1850. The second tubeenclosure 2420 communicates with the chamber interior through a pair ofopenings 2430, 2440 through the ceiling 110 and includes an insulatinggap 2452. A pair of side coil windings 2450, 2460 along the sides of thesecond tube enclosure 2420 maintain a plasma therein and are driven by asecond RF power supply 2470 through an impedance match circuit 2480. Asindicated in FIG. 24, the two orthogonal plasma currents coincide overthe wafer surface and provide more uniform coverage of plasma over thewafer surface. This embodiment is expected to find particularlyadvantageous use for processing large wafers of diameters of 10 inchesand greater.

As in the embodiment of FIG. 17, the embodiment of FIG. 24 creates twomutually orthogonal torroidal plasma current paths over the wafer 120for enhanced uniformity. The two orthogonal torroidal or closed pathsare separate and independently powered as illustrated, but intersect inthe process region overlying the wafer, and otherwise do not interact orotherwise divert or diffuse one another. In order to assure separatecontrol of the plasma source power applied to each one of the orthogonalpaths, the frequency of the respective RF generators 180, 2470 of FIG.24 are different, so that the operation of the impedance match circuits175, 2480 is decoupled. For example, the RF generator 180 may produce anRF signal at 11 MHz while the RF generator 2470 may produce an RF signalat 12 MHz. Alternatively, independent operation may be achieved byoffsetting the phases of the two RF generators 180, 2470.

FIG. 26 illustrates a variation of the embodiment of FIG. 18 in which amodified rectangular enclosure 2650 that includes an insulating gap 2658communicates with the chamber interior through the chamber side wall 105rather than through the ceiling 110. For this purpose, the rectangularenclosure 2650 has a horizontal top section 2652, a pair of downwardlyextending legs 2654 at respective ends of the top section 2652 and apair of horizontal inwardly extending legs 2656 each extending from thebottom end of a respective one of the downwardly extending legs 2654 toa respective opening 2670, 2680 in the side wall 105.

FIG. 27 illustrates how a second rectangular tube enclosure 2710including an insulating gap 2752 may be added to the embodiment of FIG.26, the second tube enclosure 2710 being identical to the rectangulartube enclosure 2650 of FIG. 26 except that the rectangular tubeenclosures 2650, 2710 are mutually orthogonal (or at least transverse toone another). The second rectangular tube enclosure communicates withthe chamber interior through respective openings through the side wall105, including the opening 2720. Like the embodiment of FIG. 25, thetube enclosures 2650 and 2710 produce mutually orthogonal torroidalplasma currents that coincide over the wafer surface to provide superioruniformity over a broader wafer diameter. Plasma source power is appliedto the interior of the tube enclosures through the respective pairs ofside coil windings 165, 185 and 2450, 2460.

FIG. 28A illustrates how the side coils 165, 185, 2450, 2460 may bereplaced (or supplemented) by a pair of mutually orthogonal interiorcoils 2820, 2840 lying within the external region 2650 surrounded by thetwo rectangular tube enclosures 2650, 2710. Each one of the coils 2820,2840 produces the torroidal plasma current in a corresponding one of therectangular tube enclosures 2650, 2710. The coils 2820, 2840 may bedriven completely independently at different frequencies or at the samefrequency with the same or a different phase. Or, they may be driven atthe same frequency but with a phase difference (i.e., 90 degrees) thatcauses the combined torroidal plasma current to rotate at the sourcepower frequency. In this case the coils 2820, 2840 are driven with thesin and cosine components, respectively, of a common signal generator2880, as indicated in FIG. 28A. The advantage is that the plasma currentpath rotates azimuthally across the wafer surface at a rotationalfrequency that exceeds the plasma ion frequency so that non-uniformitiesare better suppressed than in prior art methods such as MERIE reactorsin which the rotation is at a much lower frequency.

Referring now to FIG. 28B, radial adjustment of plasma ion density maybe generally provided by provision of a pair of magnetic cylindricalcores 2892, 2894 that may be axially moved toward or away from oneanother within the coil 2820 and a pair of magnetic cylindrical cores2896, 2898 that may be axially moved toward or away from one anotherwithin the coil 2840. As each pair of cores is moved toward one another,inductive coupling near the center of each of the orthogonal plasmacurrents is enhanced relative to the edge of the current, so that plasmadensity at the wafer center is generally enhanced. Thus, thecenter-to-edge plasma ion density may be controlled by moving the cores2892, 2894, 2896, 2898.

FIG. 29 illustrates an alternative embodiment of the invention in whichthe two tube enclosures 2650, 2710 are merged together into a singleenclosure 2910 that extends 360 degrees around the center axis of thereactor that constitutes a single plenum. In the embodiment of FIG. 29,the plenum 2910 has a half-dome lower wall 2920 and a half-dome upperwall 2930 generally congruent with the lower wall 2920. The plenum 2910is therefore the space between the upper and lower half-dome walls 2920,2930. An insulating gap 2921 may extend around the upper dome wall 2920and/or an insulating gap 2931 may extend around the lower dome wall2930. The plenum 2910 communicates with the chamber interior through anannular opening 2925 in the ceiling 110 that extends 360 degrees aroundthe axis of symmetry of the chamber.

The plenum 2910 completely encloses a region 2950 above the ceiling 110.In the embodiment of FIG. 29, plasma source power is coupled into theinterior of the plenum 2910 by a pair of mutually orthogonal coils 2960,2965. Access to the coils 2960, 2965 is provided through a verticalconduit 2980 passing through the center of the plenum 2910. Preferably,the coils 2960, 2965 are driven in quadrature as in the embodiment ofFIG. 28 to achieve an azimuthally circulating torroidal plasma current(i.e., a plasma current circulating within the plane of the wafer. Therotation frequency is the frequency of the applied RF power.Alternatively, the coils 2960, 2965 may be driven separately atdifferent frequencies. FIG. 30 is a top sectional view of the embodimentof FIG. 29. FIGS. 31A and 31B are front and side sectional views,respectively, corresponding to FIG. 30.

The pair of mutually orthogonal coils 2960, 2965 may be replaced by anynumber n of separately driven coils with their winding axes disposed at360/n degrees apart. For example, FIG. 32 illustrates the case where thetwo coils 2960, 2965 are replace by three coils 3210, 3220, 3230 withwinding axes placed at 120 degree intervals and driven by threerespective RF supplies 3240, 3250, 3260 through respective impedancematch circuits 3241, 3251, 3261. In order to produce a rotatingtorroidal plasma current, the three windings 3210, 3220, 3230 are driven120 degrees out of phase from a common power source 3310 as illustratedin FIG. 33. The embodiments of FIGS. 32 and 33 are preferred over theembodiment of FIG. 29 having only two coils, since it is felt much ofthe mutual coupling between coils would be around rather than throughthe vertical conduit 2980.

FIG. 34 illustrates an embodiment in which the three coils are outsideof the enclosed region 2950, while their inductances are coupled intothe enclosed region 2950 by respective vertical magnetic cores 3410extending through the conduit 2980. Each core 3410 has one end extendingabove the conduit 2980 around which a respective one of the coils 3210,3220, 3230 is wound. The bottom of each core 3410 is inside the enclosedregion 2950 and has a horizontal leg. The horizontal legs of the threecores 3410 are oriented at 120 degree intervals to provide inductivecoupling to the interior of the plenum 2910 similar to that provided bythe three coils inside the enclosed region as in FIG. 32.

The advantage of the flattened rectangular tube enclosures of theembodiments of FIGS. 18-28 is that the broad width and relatively lowheight of the tube enclosure forces the torroidal plasma current to be awide thin belt of plasma that more readily covers the entire surface ofa large diameter wafer. The entirety of the tube enclosure need not beof the maximum width. Instead the outer section of the tube enclosurefarthest from the chamber interior may be necked down, as discussedabove with reference to the embodiment of FIG. 20. In this case, it ispreferable to provide focusing magnets 1870 at the transition cornersbetween the wide portion 1851 and the narrow section 1864 to force theplasma current exiting the narrow portion 1864 to spread entirely acrossthe entire width of the wide section 1851. If it is desired to maximizeplasma ion density at the wafer surface, then it is preferred that thecrosssectional area of the narrow portion 1864 be at least nearly asgreat as the cross-sectional area of the wide portion 1851. For example,the narrow portion 1864 may be a passageway whose height and width areabout the same while the wide portion 1851 may have a height that isless than its width.

The various embodiments described herein with air-core coils (i.e.,coils without a magnetic core) may instead employ magnetic-cores, whichcan be the open-magnetic-path type (“rod” type cores) or theclosed-magnetic-core type illustrated in the accompanying drawings.Furthermore, the various embodiments described herein having two or moretorroidal paths driven with different RF frequencies may instead bedriven with same frequency, and with the same or different phases.

FIG. 35 is a version of the embodiment of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the embodiment of FIG. 20.

FIG. 36 is a version of the embodiment of FIG. 24 but employing a pairof magnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

FIG. 37 is an embodiment corresponding to that of FIG. 35 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber. Having a number of symmetrically disposed conduitsand reentrant ports greater than two (as in the embodiment of FIG. 37)is believed to be particularly advantageous for processing wafers ofdiameter of 300 mm and greater.

FIG. 38 is an embodiment corresponding to that of FIG. 38 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

FIG. 39 is an embodiment corresponding to that of FIG. 35 in which theexternal conduits join together in a common plenum 3910.

FIG. 40 is an embodiment corresponding to that of FIG. 36 in which theexternal conduits join together in a common plenum 4010.

FIG. 41 is an embodiment corresponding to that of FIG. 37 in which theexternal conduits join together in a common plenum 4110.

FIG. 42 is an embodiment corresponding to that of FIG. 38 in which theexternal conduits join together in a common plenum 4210.

FIG. 43 is an embodiment corresponding to that of FIG. 17 in which theexternal conduits join together in a common plenum 4310.

In the embodiments described above, such as FIG. 24 for example, two (ormore) mutually transverse torroidal reentrant plasma paths areestablished using two (or more) mutually transverse external reentrantconduits. In each of the two external reentrant conduits in FIG. 24, forexample, RF power is applied to RF source power applicators. It isdesirable that the RF power currents applied to the two RF source powerapplicators be of either different frequency or different phase from oneanother. Otherwise, an attractive RF potential between adjacent conduitscan arise periodically (or continuously) to divert plasma from oneconduit opening to the opening of an adjacent conduit, thus distortingthe plasma current path from the desired torroidal shape. Therefore,each RF source power applicator is best powered by a different RFgenerator, each RF generator having a frequency or phase different fromthe other RF generator. In addition, a third RF generator is employed toprovide plasma bias RF power to the wafer support pedestal. However, thethree RF power generators, along with the associated impedance matchingcircuitry, represents a significant cost.

FIGS. 44A and 44B illustrate an embodiment like that of FIG. 24 having apair of mutually transverse reentrant external conduits with respectiveplasma source power applicators. However, in the embodiment of FIGS. 44Aand 44B, only a single RF source power generator is needed to power thesource power applicators of the pair of external reentrant conduits. Thereactor of FIGS. 44A, 44B includes a vacuum chamber 5010 having acylindrical side wall 5015, a ceiling 5020 and a wafer support pedestal5025 within the chamber 5010. A floor 5030 of the chamber 5010 has apumping port 5035 coupled to a vacuum pump 5040. A gas distributionplate or showerhead 5045 supported on the interior surface of theceiling 5020 overlies the wafer support pedestal 5025 and has a gasdistribution manifold 5045 a and an array of gas injection orifices 5045b. A gas supply 5050 is coupled via a gas supply line 5055 to theshowerhead manifold 5045 a. A first reentrant torroidal plasma currentpath 5060 is established by a first external reentrant hollow conduit5065, the conduit 5065 having a plasma source power applicator 5068. Asecond reentrant torroidal plasma current path 5070 is established by asecond external reentrant hollow conduit 5075, the second conduit 5075having its own plasma source power applicator 5078. The ends of thehollow conduits 5065, 5075 are coupled to the interior of the chamber5010 through respective ports 5020 a, 5020 b, 5020 c, 5020 d through theceiling 5020. As in other embodiments described above in thisspecification, each conduit is formed of a conductive material and has anarrow annular gap 5077 formed of an insulating material.

The first plasma source power applicator 5068 consists of an annularferrite core 5080 surrounding a portion of the first conduit 5065 and anRF-driven conductive winding 5085 surrounding a portion of the core5080. The second plasma source power applicator 5078 has the samestructure and consists of an annular ferrite core 5090 surrounding aportion of the second conduit 5075 and an RF-driven conductive winding5095 surrounding a portion of the core 5090. Impedance matching can beoptimized by providing for each ferrite core 5080, 5090 a secondarywinding 5087, 5097 connected across a tuning capacitor 5088, 5098,respectively, each tuning capacitor preferably being variable. A plasmabias RF power generator 5100 is coupled through an impedance matchcircuit 5105 to the wafer support pedestal 5025.

As described previously above, it is necessary to keep the RF currentsthrough the two windings 5085, 5095 out of phase to avoid creatingattractive potentials between adjacent conduits 5065, 5075 that woulddivert plasma current away from the desired torroidal path. This isreadily accomplished by driving the different windings 5085, 5095 withdifferent RF generators. However, it would be a great cost savings if asingle RF generator could be employed to drive both windings 5085, 5095in such a manner as to avoid plasma flow from one conduit to the other.

This problem is solved by employing a switching circuit that divideseach cycle of the RF output current of a single RF generator 5110 intotwo different time segments and applies the different time segments ofthe current to the different windings 5085, 5095. In this way, thecurrents in each of the windings 5085, 5095 are never in phase or ofopposite phase so that the plasma currents are not diverted from theirtorroidal reentrant paths 5060, 5070. The switching circuit in FIG. 44Aconsists of a pair of high current semiconductor diodes 5120, 5125connected in opposite polarity between the RF generator 5110 andrespective ones of the windings 5085, 5095. In FIGS. 44A and 44B, thediodes 5120, 5125 are connected to the RF generator 5110 through atransformer 5130 consisting of a primary winding 5132 connected acrossthe generator 5110 and a secondary winding 5134 having a grounded centertap 5136, the secondary winding 5134 having a pair of ends connected tothe respective diodes 5120, 5125. The diodes 5120, 5125 are connected inopposite polarity, one of them having its anode connected to thesecondary winding 5134 and the other having its cathode connected to thesecondary winding 5134. Thus, each RF-driven winding 5085, 5095 isconnected (through the respective one of the two diodes 5120, 5125)across a different portion of the secondary winding 5134, one portionextending from the secondary winding end 5134 a to the grounded centertap 5136 and the other portion extending from the secondary winding end5134 b to the grounded center tap 5136. Because the diodes 5120, 5125are connected in opposite polarity to the secondary winding 5134,current supplied to the windings 5085, 5095 is derived from respectivehalf-wave rectified currents that are 180 degrees out of phase, asillustrated in FIGS. 45A and 45B representing current flow throughrespective ones of the two diodes 5085, 5095. The out-of-phaserelationship between the currents in the two windings 5085, 5095prevents plasma current flow between the two opposing conduits 5065,5075. It should be noted that the abrupt transitions in current in eachof the rectified waveforms of FIGS. 45A and 45B (due to the diodeswitching) are smoothed by the reactance associated with the RF-drivenwindings 5085, 5095, the ferrite cores 5080, 5090 and the secondarywindings 5087, 5097 and tuning capacitors 5088, 5098.

FIG. 46 illustrates a modification of the reactor of FIG. 44A in whichthe diodes 5120, 5125 are replaced by a pair of metal oxidesemiconductor field effect transistors (MOSFETs) 5142, 5144 whose gatesare controlled by complementary clock signals (FIGS. 47B and 47C,respectively) synchronized with the sinusoidal current from the RFgenerator 5110 (FIG. 47A). In one implementation, a clock signalgenerator 5140 is synchronized with the RF generator 5110 and is coupleddirectly to the gate of the MOSFET 5144 and through an inverter 5145 tothe gate of the other MOSFET 5142. The RF generator output current isillustrated in FIG. 47A and the complementary clock signals areillustrated in FIGS. 47B and 47C. The resulting RF currents in thewindings 5085, 5095 are as shown in FIGS. 45A and 45B.

FIG. 48 illustrates a modification of the embodiment of FIG. 44A inwhich the transformer 5130 is eliminated and the RF generator 5110 isconnected directly to the diodes 5120, 5125. FIG. 49 illustrates amodification of the embodiment of FIG. 48 in which higher currents areaccommodated by providing several diodes in parallel to switch currentto each of the windings 5085, 5095. Thus, the diode 5120 is replaced bya set of parallel diodes 5120 a-d and the diode 5125 is replaced by theset of parallel diodes 5125 a-d. One can use existing integratedrectifier modules instead of discrete diodes to achieve the goal ofparallel paths (higher currents). Integrated diodes have minimuminductances and can switch very fast.

FIGS. 50A and 50B illustrate a plasma reactor in which the vacuumchamber 5010 houses two wafer support pedestals 5025-1 and 5025-2. Twooverhead gas distribution plates 5045-1 and 5045-2 supported on theinterior surface of the ceiling 5020 overlie respective ones of thewafer support pedestals 5025-1, 5025-2 to establish plasma processingzones 5150-1, 5150-2 over the wafer support pedestals 5025-1, 5025-2,respectively. A pair of mutually transverse reentrant plasma currentpaths are established through the processing zone 5150-1 by a pair ofmutually transverse external reentrant hollow conduits 5165-1, 5175-1whose respective ends mate with respective ceiling ports 5020-1 a,5020-1 b, 5020-1 c, 5020-1 d. Similarly, a pair of mutually transversereentrant plasma current paths are established through the otherprocessing zone 5150-2 by a pair of mutually transverse externalreentrant hollow conduits 5165-2, 5175-2 whose respective ends mate withrespective ceiling ports 5020-2 a, 5020-2 b, 5020-2 c, 5020-2 d. Aplasma source power applicator 5068 on the conduit 5065-1 has a ferritecore 5080-1 and a core winding 5085-1. A plasma source power applicator5078-1 on the conduit 5075-1 has a ferrite core 5090-1 and a corewinding 5095-1. A plasma source power applicator 5068-2 on the conduit5065-2 has a ferrite core 5080-2 and a core winding 5085-2. A plasmasource power applicator 5078-2 on the conduit 5075-2 has a ferrite core5090-2 and a core winding 5095-2. Thus, there are four core windingsthat can be driven with four different phases to avoid creating plasmacurrents between adjacent conduits that would otherwise detract from thedesired torroidal plasma currents, as explained previously herein. Asimple way to accomplish this would be to drive the four core windings5085-1, 5095-1, 5085-2, 5095-2 with four different RF power generatorsset at four different frequencies or four different phases. In addition,two RF generators would be employed to apply separate RF bias power tothe two wafer support pedestals 5025-1, 5025-2, for a total of six RFpower generators.

In order to avoid the expense of four RF generators to drive four corewindings, the embodiment of FIG. 51 employs only a single RF generator5310 and a four-way electronic switching network 5320. The switchingnetwork 5320 divides each cycle of the sinusoidal output current of theRF generator 5310 (shown in FIG. 52 into four time segments (labeled A,B, C, D in FIG. 52) to produce four separate RF output currents shown inFIGS. 53A, 53B, 53C, 53D respectively. The switching network 5320separately applies the four output currents to the four core windings5085-1, 5095-1, 5085-2, 5095-2. One example of such a four-way switchingnetwork is illustrated in FIG. 54 in which four MOSFETs 5610 a-5610 dare connected between the RF generator 5310 and respective ones of thefour core windings 5085-1, 5095-1, 5085-2, 5095-2, the gates of theMOSFETs 5610 a-5610 d being driven in synchronism with the RF generatoroutput current (FIG. 55) by four respective mutually exclusive clocksignals CLK A, CLK B, CLK C, CLK D of FIGS. 56A, 56B, 56C, 56Drespectively.

In order to avoid the additional expense of two RF generators to applyplasma bias power to the two wafer pedestals 5025-1, 5025-2, a two-wayswitching network 5630 connected between a single bias power generator5640 and the two pedestals 5025-1, 5025-2 can apply different timesegments of each cycle of the output of the bias generator 5640 todifferent ones of the two pedestals 5025-1, 5025-2. For example, thepositive half-cycle is applied to one pedestal 5025-1 while the negativehalf cycle is applied to the other pedestal 5025-2.

FIG. 57 illustrates an embodiment corresponding to FIG. 50A in which twoRF generators 5710, 5720 of different frequencies drive the four corewindings using a diode-controlled current divider 5730. Thediode-controlled current divider 5730 includes a first pair diodes 5740,5745 connected in opposite polarity to the output of the first RFgenerator 5710 (so that the anode of one and the cathode of the other isconnected to the RF generator 5710). The remaining (“output”) ends ofthe diodes 5740, 5745 are connected to core windings associated withdifferent processing regions 5150-1, 5150-2. Specifically, the outputend of the diode 5740 drives the core winding 5085-1 while the outputend of the diode 5745 drives the core winding 5085-2. Similarly, asecond pair of diodes 5750, 5755 are connected in opposite polarity tothe output of the second RF generator 5720 (so that the anode of one andthe cathode of the other is connected to the RF generator 5720). Theremaining (“output”) ends of the diodes 5750, 5755 are connected toremaining core windings associated with the different processing regions5150-1, 5150-2. Specifically, the output end of the diode 5750 isconnected to the core winding 5095-2 while the output end of the diode5755 is connected to the core winding 5095-1.

Operation of the diode-controller current divider 5730 may be understoodwith reference to the contemporaneous timing diagrams of FIGS. 58A, 58B,59A, 59B and 60A, 60B. FIGS. 58A and 58B illustrate the output currentsof the two RF generators 5710, 5720, which are of different frequenciesor different phases. FIGS. 59A and 59B illustrate the RF currentsapplied to the core windings 5085-1, 5095-1, respectively, of the firstprocessing region 5150-1. FIGS. 60A and 60B illustrate the RF currentsapplied to the core windings 5085-2, 5095-2, respectively, of the secondprocessing region 5150-2. These figures show how the RF currents of eachpair of mutually transverse conduits are out of phase so as to avoidestablishing an attractive potential between the two plasma currents ofa given processing region.

In the embodiment of FIG. 57, the two-way bias power switching network5630 of FIG. 51 is implemented as a diode-controlled current divider,consisting of a pair of diodes 5810, 5820 connected in oppositepolarities between the common bias power generator 5640, and the twowafer support pedestals 5025-1, 5025-2. In addition, an impedance matchcircuit 5830 may be connected at the output of the bias power generator5640.

While the embodiments described in FIGS. 44A through 60 employ a plasmasource power applicator at each external conduit consisting of a ferriteor magnetic annular core surrounding a belt portion of the conduit andan RF-driven winding around the core, other types of plasma RF sourcepower applicators may be used. For example, the plasma source powerapplicators of other embodiments described in this specification employRF-driven conductor windings wound directly around the hollow conduititself (without requiring a magnetic core) or windings wound paralleland alongside the hollow conduit. Such plasma source power applicatorsmay be used as well in the embodiments of FIGS. 44A through 60.

In the embodiments of FIGS. 44A through 60, impedance matching isfacilitated by a secondary winding around the core connected across atuning capacitor (e.g., in FIG. 44A, the secondary winding 5087 andtuning capacitor 5088 as well as the secondary winding 5097 and tuningcapacitor 5098). However, other impedance matching techniques may beemployed in addition to or instead of the tuning capacitor 5088 andsecondary winding 5087. Such other techniques may include frequencytuning at the RF generator (e.g., at the RF generator 5110) or an activematch circuit at each of the RF power applicators (e.g., an activeimpedance match circuit connected to the source power applicator 5068 ofFIG. 44A and another active impedance match circuit connected to thesource power applicator 5078 of FIG. 44A).

Advantageous Features of the Invention:

The reactor of the invention affords numerous opportunities forincreasing etch selectivity without sacrificing other performancefeatures such as etch rate. For example, constricting the torroidalplasma current in the vicinity of the wafer not only improves etchselectivity but at the same time increases the etch rate by increasingthe plasma ion density. It is believed no prior reactor has increasedetch selectivity by the same mechanism that increases etch rate orplasma ion density over the workpiece.

Improving etch selectivity by constricting the torroidal plasma currentin the vicinity of the wafer or workpiece can be achieved in theinvention in any one of several ways. One way is to reduce thepedestal-to-ceiling or wafer-to-ceiling height. Another is to introducea gas distribution plate or showerhead over the wafer that constrictsthe path of the torroidal plasma ion current. Another way is to increasethe RF bias power applied to the wafer or workpiece. Any one or anycombination of the foregoing ways of improving etch selectivity may bechosen by the skilled worker in carrying out the invention.

Etch selectivity may be further improved in the invention by injectingthe reactive process gases locally (i.e., near the wafer or workpiece)while injecting an inert diluent gas (e.g., Argon) remotely (i.e., intothe conduit or plenum). This is preferably accomplished by providing agas distribution plate or showerhead directly over and facing theworkpiece support and introducing the reactive process gas exclusively(or at least predominantly) through the showerhead. Concurrently, thediluent gas is injected into the conduit well away from the processregion overlying the wafer or workpiece. The torroidal plasma currentthus becomes not only a source of plasma ions for reactive ion etchingof materials on the wafer but, in addition, becomes an agent forsweeping away the reactive process gas species and theirplasma-dissociated progeny before the plasma-induced dissociationprocess is carried out to the point of creating an undesirable amount offree fluorine. This reduction in the residence time of the reactiveprocess gas species enhances the etch selectivity relative tophotoresist and other materials, a significant advantage.

The invention provides great flexibility in the application of RF plasmasource power to the torroidal plasma current. As discussed above, poweris typically inductively coupled to the torroidal plasma current by anantenna. In many embodiments, the antenna predominantly is coupled tothe external conduit or plenum by being close or next to it. Forexample, a coil antenna may extend alongside the conduit or plenum.However, in other embodiments the antenna is confined to the regionenclosed between the conduit or plenum and the main reactor enclosure(e.g., the ceiling). In the latter case, the antenna may be consideredto be “under” the conduit rather than alongside of it. Even greaterflexibility is afford by embodiments having a magnetic core (or cores)extending through the enclosed region (between the conduit and the mainchamber enclosure) and having an extension beyond the enclosed region,the antenna being wound around the core's extension. In this embodimentthe antenna is inductively coupled via the magnetic core and thereforeneed not be adjacent the torroidal plasma current in the conduit. In onesuch embodiment, a closed magnetic core is employed and the antenna iswrapped around the section of the core that is furthest away from thetorroidal plasma current or the conduit. Therefore, in effect, theantenna may be located almost anywhere, such as a location entirelyremote from the plasma chamber, by remotely coupling it to the torroidalplasma current via a magnetic core.

Finally, the invention provides uniform coverage of the plasma over thesurface of a very large diameter wafer or workpiece. This isaccomplished in one embodiment by shaping the torroidal plasma currentas a broad plasma belt having a width preferably exceeding that of thewafer. In another embodiment, uniformity of plasma ion density acrossthe wafer surface is achieved by providing two or more mutuallytransverse or orthogonal torroidal plasma currents that intersect in theprocess region over the wafer. The torroidal plasma currents flow indirections mutually offset from one another by 360/n. Each of thetorroidal plasma currents may be shaped as a broad belt of plasma tocover a very large diameter wafer. Each one of the torroidal plasmacurrents may be powered by a separate coil antenna aligned along thedirection of the one torroidal plasma current. In one preferredembodiment, uniformity is enhanced by applying RF signals of differentphases to the respective coil antennas so as to achieve a rotatingtorroidal plasma current in the process region overlying the wafer. Inthis preferred embodiment, the optimum structure is one in which thetorroidal plasma current flows in a circularly continuous plenumcommunicating with the main chamber portion through a circularlycontinuous annular opening in the ceiling or side wall. This latterfeature allows the entire torroidal plasma current to rotate azimuthallyin a continuous manner.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

1. A plasma reactor for processing a workpiece, said plasma reactorcomprising: an enclosure; a workpiece support within the enclosurefacing an overlying portion of the enclosure, said workpiece support andthe overlying portion of said enclosure defining a process regiontherebetween extending generally across the diameter of said wafersupport; said enclosure having a first and second pairs of openingstherethrough, the two openings of each of said first and second pairsbeing at generally opposite sides of said workpiece support; a firsthollow conduit outside of said process region and connected to saidfirst pair of openings, providing a first torroidal path extendingthrough said conduit and across said process region; a second hollowconduit outside of said process region and connected to said second pairof openings, providing a second torroidal path extending through saidconduit and across said process region; first and second plasma sourcepower applicators inductively coupled to the interiors of said first andsecond hollow conduits, respectively, each of said first and secondplasma source power applicators being capable of maintaining a plasma ina respective one of said first and second torroidal paths; an RF powergenerator providing an RE output current; a current switching networkconnected between said RF power generator and said first and secondplasma source power applicators for applying respective periodic timesegments of RF output current to respective ones of said first andsecond plasma source power applicators.
 2. The plasma reactor of claim 1wherein said current switching network comprises a diode-controlledcurrent divider.
 3. The plasma reactor of claim 2 wherein saiddiode-controlled current divider comprises a first diode connectedbetween said RF power generator and said first plasma source powerapplicator in a first diode polarity and a second diode connectedbetween said RF power generator and said second plasma source powerapplicator in a second diode polarity opposite said first diodepolarity.
 4. The plasma reactor of claim 1 wherein said currentswitching network comprises respective transistor switches separatelyconnected between said RF power generator and respective ones of saidfirst and second plasma source power applicators and a source ofmutually exclusive enabling signals controlling said respectivetransistor switches.
 5. The plasma reactor of claim 4 wherein saidenabling signals and said RF power generator are synchronized.
 6. Theplasma reactor of claim 1 wherein each one of said plasma source powerapplicators comprises an RF-driven conductive winding adjacent thecorresponding one of said first and second conduits.
 7. The plasmareactor of claim 6 wherein each one of said plasma source powerapplicators further comprises an annular magnetic core surrounding anannular portion of the corresponding conduit, said RF-driven conductivewinding being wound around a portion of said annular magnetic core. 8.The plasma reactor of claim 7 wherein each one of said plasma sourcepower applicators further comprises a secondary conductive winding woundaround another portion of said annular magnetic core and a tuningcapacitor connected across said secondary winding.
 9. The plasma reactorof claim 1 wherein each one of said conduits is formed of a metalmaterial, each said conduit having an insulating gap within a wall ofthe conduit extending transversely to the corresponding torroidal pathand separating the conduit into two portions so as to prevent formationof a closed electrical path along the length of the conduit.
 10. Theplasma reactor of claim 6 wherein the RF driven winding is juxtaposedalong the corresponding conduit.
 11. The plasma reactor of claim 1wherein said first and second conduits are mutually transverse andwherein said first and second torroidal paths intersect in saidprocessing region.
 12. The plasma reactor of claim 11 wherein said fistand second conduits are mutually orthogonal.
 13. The plasma reactor ofclaim 1 wherein said first and second pairs of openings are through saidceiling.
 14. The plasma reactor of claim 1 further comprising a biaspower RF generator coupled to said wafer support pedestal.
 15. A plasmareactor for processing a workpiece, said plasma reactor comprising: anenclosure; a workpiece support within the enclosure facing an overlyingportion of the enclosure, said workpiece support and the overlyingportion of said enclosure defining a process region therebetweenextending generally across the diameter of said wafer support; an arrayof pairs of openings through said vacuum enclosure, the two openings ofeach pair being at generally opposite sides of said workpiece support;an array of hollow conduits outside of said vacuum chamber, andconnected to respective ones of said pairs of openings, whereby toprovide respective closed torroidal paths for plasma, each of saidrespective closed torroidal paths extending outside of said vacuumchamber through a respective one of said array of conduits and extendinginside said vacuum chamber between a respective pair of said openingsthrough said process region; respective source power applicators nearrespective ones of said conduits; a source power RF generator having acyclical current output; and a current switching network connectedbetween said source power RF generator and each of said respectivesource power applicators, for dividing each cycle of said cyclicalcurrent output into respective time segments and applying saidrespective time segments to respective ones of said source powerapplicators.
 16. The plasma reactor of claim 15 wherein said currentswitching network comprises diode-controlled current divider.
 17. Theplasma reactor of claim 15 wherein said current switching networkcomprises respective transistor switches separately connected betweensaid RF power generator and respective ones of said plasma source powerapplicators and a source of mutually exclusive enabling signalscontrolling said respective transistor switches.
 18. The plasma reactorof claim 17 wherein said mutually exclusive enabling signals and said RFpower generator are synchronized.
 19. The plasma reactor of claim 15wherein each one of said plasma source power applicators comprises anRF-driven conductive winding.
 20. The plasma reactor of claim 19 whereineach one of said plasma source power applicators further comprises anannular magnetic core surrounding an annular portion of thecorresponding conduit, said RF-driven conductive winding being woundaround a portion of said annular magnetic core.
 21. The plasma reactorof claim 20 wherein each one of said plasma source power applicatorsfurther comprises a secondary conductive winding wound around anotherportion of said annular magnetic core and a tuning capacitor connectedacross said secondary winding.
 22. The plasma reactor of claim 15wherein each one of said conduits is formed of a metal material, eachsaid conduit having an insulating gap within a wall of the conduitextending transversely to the corresponding torroidal path andseparating the conduit into two portions so as to prevent formation of aclosed electrical path along the length of the conduit.
 23. The plasmareactor of claim 19 wherein the RF driven winding is juxtaposed alongthe corresponding conduit.
 24. The plasma reactor of claim 15 whereinsaid conduits are mutually transverse and wherein said torroidal pathsintersect in said processing region.
 25. The plasma reactor of claim 15wherein said pairs of openings are through said ceiling.
 26. The plasmareactor of claim 15 further comprising a bias power RF generator coupledto said wafer support pedestal.
 27. The reactor of claim 15 wherein eachconduit has a width along an axis parallel with the plane of said wafersupport which is at least as great as the diameter of said wafersupport.
 28. The reactor of claim 27 wherein each conduit has a heightalong an axis perpendicular to the plane of said wafer support which isless than said width.
 29. The reactor of claim 28 wherein each conduithas a rectangular cross-section whereby to produce a relatively thinwide belt of plasma in said closed torroidal path.
 30. The reactor ofclaim 20 further comprising a transformer connected between said RFgenerator and the respective plasma source power applicators, saidtransformer comprising a primary winding connected across said RFgenerator and a secondary winding having respective portions thereofconnected through said switching network across respective ones of saidplasma source power applicators.
 31. A plasma reactor for processing aworkpiece, said plasma reactor comprising: an enclosure; a workpiecesupport within the enclosure facing an overlying portion of theenclosure, said workpiece support and the overlying portion of saidenclosure defining a process region therebetween extending generallyacross the diameter of said wafer support; said enclosure having atleast first and second pairs of openings therethrough, the openings ofeach pair being at generally opposite sides of said workpiece support;at least first and second hollow conduits outside of said process regionand connected between respective ones of said first and second pairsopenings, providing first and second torroidal paths extending throughsaid conduit and across said process region; respective plasma sourcepower applicators adjacent respective ones of said conduits formaintaining a plasma in each torroidal path; a source power RF generatorhaving a cyclical current output; and a current switching networkconnected between said source power RF generator and each of saidrespective source power applicators, for dividing each cycle of saidcyclical current output into respective time segments and applying saidrespective time segments to respective ones of said source powerapplicators; wherein the height of said closed torroidal path along anaxis generally perpendicular to a plane of said wafer support in aprocess region overlying said workpiece support is less than elsewherein said closed torroidal path, whereby to enhance the plasma ion densityin said process region relative to the plasma ion density elsewhere insaid closed torroidal path.
 32. The plasma reactor of claim 31 furthercomprising a conductive body between said workpiece support and saidvacuum enclosure and constricting said torroidal paths in said processregion overlying said wafer support.
 33. The plasma reactor of claim 32further comprising an RF bias power supply coupled to said wafer supportand capable of maintaining a plasma sheath over a workpiece on saidworkpiece support of a thickness which constricts said torroidal pathswithin said process region so as to enhance plasma ion density in saidprocess region overlying said workpiece support.
 34. The plasma reactorof claim 31 wherein said conduit is formed of a metal material, saidconduit having an insulating gap within a wall of the conduit extendingtransversely to said torroidal path and separating said conduit into twoportions so as to prevent formation of a closed electrical path alongthe length of said conduit.
 35. The plasma reactor of claim 31 whereinsaid vacuum enclosure comprises a longitude side wall and an overlyinglateral ceiling, and wherein said first and second pairs of openingsextend through said ceiling.
 36. The plasma reactor of claim 32 whereinsaid conductive body comprises a gas distribution plate, and wherein theheight of said closed torroidal path along an axis perpendicular to aplane of said wafer support is the distance between said gasdistribution plate and said wafer support.
 37. The plasma reactor ofclaim 31 wherein said current switching network comprises adiode-controlled current divider.
 38. The plasma reactor of claim 31wherein said current switching network comprises respective transistorswitches separately connected between said RF power generator andrespective ones of said first and second plasma source power applicatorsand a source of mutually exclusive enabling signals controlling saidrespective transistor switches.
 39. The plasma reactor of claim 31wherein each one of said plasma source power applicators comprises anRF-driven conductive winding near the corresponding one of said firstand second conduits.
 40. The plasma reactor of claim 39 wherein eachplasma source power applicator further comprises an annular magneticcore surrounding an annular portion of the corresponding conduit, saidRF-driven conductive winding being wound around a portion of saidannular magnetic core.
 41. The plasma reactor of claim 40 wherein eachone of said plasma source power applicators further comprises asecondary conductive winding wound around another portion of saidannular magnetic core and a tuning capacitor connected across saidsecondary winding.
 42. A plasma reactor for processing a workpiece, saidplasma reactor comprising: an enclosure; plural workpiece supportswithin the enclosure facing overlying portions of the enclosure, saidworkpiece supports and respective ones of the overlying portions of saidenclosure defining respective plural process regions therebetweenextending generally across the diameters of respective ones of saidwafer supports; plural arrays of pairs of openings through said vacuumenclosure, each of said arrays being generally centered around arespective one of said process regions, the two openings of each pairbeing at generally opposite sides of the corresponding one of saidplural workpiece supports; plural arrays of hollow conduits outside ofsaid vacuum chamber, and connected to respective arrays of said pairs ofopenings, each array of hollow conduits providing a respective pluralityof torroidal plasma current path through a corresponding one of saidplural process regions; respective source power applicators nearrespective ones of said conduits; a source power RF generator having acyclical current output; and a current switching network connectedbetween said source power RF generator and each of said respectivesource power applicators, for dividing each cycle of said cyclicalcurrent output into respective time segments and applying saidrespective time segments to respective ones of said source powerapplicators.
 43. The plasma reactor of claim 42 wherein: said reactorcomprises a second source power RF generator having a second cyclicalcurrent output, said current switching network being connected betweenboth source power RF generators and respective ones of said source powerapplicators, for dividing the cycle of the cyclical current outputs ofboth source power RF generators into respective time segments andapplying said respective time segments to respective ones of said sourcepower applicators; each array of hollow conduits constitutes two hollowconduits; each array of pairs of openings constitutes two openings; andsaid current switching network comprises diode-controlled currentdivider.
 44. The plasma reactor of claim 42 wherein said currentswitching network comprises respective transistor switches separatelyconnected between said RF power generator and respective ones of saidplasma source power applicators and a source of mutually exclusiveenabling signals controlling said respective transistor switches. 45.The plasma reactor of claim 44 wherein said mutually exclusive enablingsignals and said RF power generator are synchronized.
 46. The plasmareactor of claim 42 wherein each one of said plasma source powerapplicators comprises an RF-driven conductive winding.
 47. The plasmareactor of claim 46 wherein each one of said plasma source powerapplicators further comprises an annular magnetic core surrounding anannular portion of the corresponding conduit, said RF-driven conductivewinding being wound around a portion of said annular magnetic core. 48.The plasma reactor of claim 47 wherein each one of said plasma sourcepower applicators further comprises a secondary conductive winding woundaround another portion of said annular magnetic core and a tuningcapacitor connected across said secondary winding.
 49. The plasmareactor of claim 42 wherein each one of said conduits is formed of ametal material, each said conduit having an insulating gap within a wallof the conduit extending transversely to the corresponding torroidalpath and separating the conduit into two portions so as to preventformation of a closed electrical path along the length of the conduit.